1. Field of the Invention
The present invention relates to an ink jet head for ejecting ink through nozzles by exerting pressure on ink in ink channels formed in the ink jet head, and an ink jet recorder including the ink jet head.
2. Description of the Related Art
A communication device such as a facsimile machine or an information processor such as a personal computer usually has a recorder capable of recording data, comprising characters or graphics, onto a sheet of paper so as to store this data as visual information. The recorder adopts a printing system such as impact system, thermal system or ink jet system. In recent years, increased attention has been paid to an ink jet recorder using the ink jet system which is highly quiet and can print on various types of paper.
The above-described ink jet recorder has an ink jet head with numerous nozzles for ejecting ink droplets toward a sheet of paper so as to print characters or graphics thereon. Usually, the ink jet head, as illustrated in FIG. 14, is constituted such that a side wall 101 of an ink channel 105 communicating with a nozzle 100 is composed of a polarized piezoelectric material, and an electrode 102 is formed on the surface of the side wall. A drive electric field is applied to the side wall 101 via the electrode 102 to bend the side wall 101 and change the capacity of the ink channel 105. The resulting increase and decrease in the ink pressure cause the ink to be discharged (ejected) from and supplied to the ink channel 105.
If air bubbles 103 are present in the ink channel 105, a change in the capacity of the ink channel 105 is accommodated by the shrinkage and expansion of the air bubbles 103. Thus, the change in the capacity of the ink channel 105 is not fully reflected in an increase or decrease in the ink pressure, so that the ejection of an ink droplet may be insufficient. If the air bubbles 103 grow, they may obstruct the ink channel 105 or the nozzle 100, resulting in an ejection failure. Hence, when printing is started or ejection failure has been detected, it has been customary practice to carry out a suction treatment for locating suction means in close proximity to the outside of the nozzle 100 and sucking the air bubbles 103 together with ink in the ink channel 105.
However, a mere purge or suction treatment as described above tends to remove the air bubbles 103 insufficiently, if the air bubbles 103 exist at the corner of the nozzle plate 104 and the ink channel 105.
An attempt at the reliable removal of the air bubbles 103 would require that such a treatment be repeated many times.
There has been a proposal for a structure in which the bottom surface of the ink channel 105 is curved upward toward the nozzle 100 and the cross-sectional area of the ink channel is decreased to smooth the flow of ink and discharge the air bubbles 103 simultaneously with ink ejection through the nozzle (Japanese Unexamined Patent Publication No. 6-171096 corresponding to U.S. Pat. No. 5,650,810). This structure always results in a mere reduction of the cross-sectional area. It has remained unclear what cross-sectional shape and what dimensions will make the air bubbles 103 fully discharged.